The present invention relates to a coated cemented carbide insert cutting tool particularly useful for dry milling of grey cast iron.
Grey cast-iron is a material which, in general, is reasonably easy to machine with cemented carbide tools. Often, long tool life can be obtained. However, the machinability of cast iron can vary considerably. The tool life may be influenced significantly by small variations in the chemical composition within the material. These variations may be related to the casting technique used, such as the cooling conditions. Other causes for variations are the casting skin and sand inclusions, if present, or even the stability of the machine used for cutting the material. It is well-known that the machinability of a cast iron may vary from one batch to another.
There is always a desire for a reliable and stable tool life in mass volume production and in particular when unmanned production is employed.
When machining grey cast iron using coated milling cutters, the cutters are worn mainly by a so called adhesive wear mechanism. That is, fragments or individual grains of the layers and later also parts of the cemented carbide are successively pulled away from the cutting edge by the chip formed. Such wear mechanism is further accelerated by the formation of cracks about 50 .mu.m apart along and perpendicular to the cutting edge generally referred to as comb cracks. They are the result of thermal fatigue caused by the intermittent cutting which is typical for all milling cutting operations. Commercial cemented carbide milling inserts therefore generally contain gamma-phase, which is a solid solution of WC--TiC--TaC--NbC, in order to decrease heat influenced wear mechanism.
As soon as the coating is worn out, the underlying cemented carbide will wear fast. This will lead to higher cutting forces. The comb cracks formed will, after some time, cause severe chipping of the cutting edge and the damaged tool will generate a bad surface or chipping of the machined component.
Chipforming operations predominantly of adhesive wear type put high demands on both the coating and the cemented carbide. A cemented carbide that can withstand comb crack formation as long as possible and a coating that adheres well to the cemented carbide is desirable. Further, it is desired that there are low adhesion forces between the chip and the coating and that the coating has a high internal strength (cohesiveness) a property which is believed to be of significant importance.